The present invention relates generally to spacecraft attitude control systems and methods, and more particularly, to systems and methods that limit the effects of actuator saturation to certain body axes of a spacecraft.
Certain spacecraft use three momentum wheels that are respectively oriented parallel to the x axis, y axis, and z axis of a spacecraft. If a requested y-axis torque exceeds the maximum torque output of the y-axis momentum wheel, then the y-axis motor driving the momentum wheel saturates, introducing a y-axis attitude error. Since the x-axis and z-axis motors are orthogonal to the y-axis, they have no immediate effect on the motion of the spacecraft about the y-axis. Therefore, adjusting the x-axis and z-axis torque cannot reduce or prevent the y-axis attitude error.
Therefore. it would be advantageous to have an arrangement of momentum wheels for use on a spacecraft and processing methods that allow actuator saturation to be managed so that saturation does not produce an attitude error about a particular axis.
The present invention provides for systems and methods that limit the effects of actuator saturation to certain body axes of a spacecraft. The conventional approach of orienting momentum wheels parallel to the respective x axis, y axis, and z axis of the spacecraft is not used in the present invention. In the present invention, the actuators, or momentum wheels, are not oriented parallel to the x, y, and z axes of the spacecraft.
When each of the three momentum wheels are not oriented parallel to the spacecraft axes, each momentum wheel affects the motion about two or more axes of the spacecraft. This orientation of the momentum wheels allows actuator saturation to be managed using methods in accordance with the present invention so that actuator saturation produces no attitude error about any particular axis.
For example, a spacecraft operating with a pitch momentum bias can tolerate roll and yaw torque error much better than it can tolerate pitch torque error. In such a spacecraft, the present invention may be used to ensure that torque errors about the pitch axis are prevented whenever possible, even if this introduces roll or yaw torque errors.
A second example is the case of a satellite which can tolerate more error about the boresight of its antenna than other rotations which move the boresight away from its target. This invention can be used to prevent torque errors orthogonal to the antenna boresight whenever possible, even if this introduces torque errors about the antenna boresight.
An exemplary embodiment of the present invention operates to prevent torque errors about the pitch axis of a spacecraft having a pitch momentum bias. Pitch momentum bias causes stiffness about roll and yaw axes, greatly reducing attitude errors caused by roll torque errors and yaw torque errors. However, since the pitch momentum bias adds no stiffness to the pitch axis, any pitch torque error results in relatively large pitch attitude errors. To prevent these large pitch attitude errors, the present invention is used to prevent wheel torque saturation from causing any pitch torque error. An algorithm is disclosed that is implemented in a processor coupled to each of the momentum wheels to implement this exemplary embodiment.
The present invention may be implemented with an arbitrary number of hierarchical levels. For example, saturation may be managed such that it affects yaw first, then roll, and finally pitch.